Currently, active matrix organic light emitting device (“AMOLED”) displays are being introduced. The advantages of such displays include lower power consumption, manufacturing flexibility and faster refresh rate over conventional liquid crystal displays. In contrast to conventional liquid crystal displays, there is no backlighting in an AMOLED display as each pixel consists of different colored OLEDs emitting light independently. The OLEDs emit light based on current supplied through a drive transistor. The drive transistor is typically a thin film transistor (TFT). The power consumed in each pixel has a direct relation with the magnitude of the generated light in that pixel.
The drive-in current of the drive transistor determines the pixel's OLED luminance. Since the pixel circuits are voltage programmable, the spatial-temporal thermal profile of the display surface changing the voltage-current characteristic of the drive transistor impacts the quality of the display. The rate of the short-time aging of the thin film transistor devices is also temperature dependent. Further the output of the pixel is affected by long term aging of the drive transistor. Proper corrections can be applied to the video stream in order to compensate for the unwanted thermal-driven visual effects. Long term aging of the drive transistor may be properly determined via calibrating the pixel against stored data of the pixel to determine the aging effects. Accurate aging data is therefore necessary throughout the lifetime of the display device.
Currently, displays having pixels are tested prior to shipping by powering all the pixels at full brightness. The array of pixels is then optically inspected to determine whether all of the pixels are functioning. However, optical inspection fails to detect electrical faults that may not manifest themselves in the output of the pixel. The baseline data for pixels is based on design parameters and characteristics of the pixels determined prior to leaving the factory but this does not account for the actual physical characteristics of the pixels in themselves.
Various compensation systems use a normal driving scheme where a video frame is always shown on the panel and the OLED and TFT circuitries are constantly under electrical stress. Moreover, pixel calibration (data replacement and measurement) of each sub-pixel occurs during each video frame by changing the grayscale value of the active sub-pixel to a desired value. This causes a visual artifact of seeing the measured sub-pixel during the calibration. It may also worsen the aging of the measured sub-pixel, since the modified grayscale level is kept on the sub-pixel for the duration of the entire frame.
Additionally, previous compensation technique for OLED displays considered backplane aging and OLED efficiency lost. The aging (and/or uniformity) of the panel was extracted and stored in lookup tables as raw or processed data. Then a compensation block used the stored data to compensate for any shift in the electrical parameters of the backplane (e.g., threshold voltage shift) or the OLED (e.g., shift in the OLED operating voltage). Such techniques can be used to compensate for OLED efficiency losses as well. These techniques are based on the assumption that the OLED color coordinates are stable despite reductions in the OLED efficiency. Depending on the OLED material and the required device lifetime, this can be a valid assumption. However, for OLED materials with low stability in color coordinates, this can result in excessive display color shifts and image sticking issues.
The color coordinates (i.e., chromaticity) of an OLED shift over time. These shifts are more pronounced in white OLEDs since the different color components that are combined in an OLED structure used to create white light can shift differently (e.g., the blue portion may age faster than the red or green portion of the combined OLED stack), leading to undesirable shifts in the display white point, which in turn lead to artifacts such as image sticking. Moreover, this phenomenon is applicable to other OLEDs as well, such as OLEds that consist of only single color components in a stack (i.e., single Red OLED stack, single GREEN OLED stack, etc.). As a result, color shifts that occur in the display can cause severe image sticking issues.
Furthermore, as discussed in previous documents and patents, IGNIS Maxlife™ can compensate for both OLED and backplane issues including aging, non-uniformity, temperature, and so on. Calculations of compensation factors is performed with dedicated resources of a display.
Therefore, there is a need for techniques to provide accurate measurement of the display temporal and spatial information and ways of applying this information to improve display uniformity in an AMOLED display. There is also a need to determine baseline measurements of pixel characteristics accurately for aging compensation purposes.